When drilling in soil, a central pilot drill bit is used together with a reamer drill bit so arranged that it can be thrown out eccentrically in relation to the pilot drill bit and expand the hole drilled by it. Thereby the bore will be of a dimension enough to give room not only for the pilot drill bit with the reamer drill bit in its inner position but also for a casing tube brought down in the hole at the same time as the hole is drilled. The casing tube is driven downwards with the drill bit through a casing shoe welded to the end of the casing tube and influenced by the percussive drill hammer. The casing tubes usually consist of three meters in length of steel welded together as drilling and the tube is driven downwards to a suitable extent.
One problem when dealing with the casing tubes is their weight. One further problem is the possibility of corrosion, which in some cases causes the exchange of them to plastic tubes after the drilling, a circumstantial and laborious procedure.